The present invention relates generally to apparatus for providing a measure of the distance between the apparatus and an object. In particular, the present invention is directed to automatic focusing systems in which a primary optical means, such as the taking lens of a camera, is moved to maintain an image of the object in focus at the plane of a photographic film.
One highly advantageous type of automatic focus apparatus is the spatial image correlation type. Examples of the different forms of arrangements of this type can be found in copending U.S. patent application Ser. No. 700,963, filed June 29, 1976, by Norman L. Stauffer, and assigned to the same assignee as the present invention; in U.S. Pat. Nos. 3,836,772; 3,838,275; 3,958,117; and No. 4,002,899; by Norman L. Stauffer, and in U.S. Pat. No. 3,274,914, by K. Biederman, et al.
In my copending application Ser. No. 743,189, filed Nov. 19, 1976, and assigned to the assignee of the present invention, I provide a continuous automatic focus system that produces an operation that occurs only with respect to the position of the major extremum to supply an accurate focus correction signal and to drive the lens in an appropriate direction to achieve proper focus. In my copending application Ser. No. 804,111, filed June 6, 1977, I provide an improved continuous automatic focus system which permits proportional control, that is, driving of the motor to position the lens at relatively high rates for large errors and reducing the speed of the motor drive at small errors to prevent overshoot and focus oscillation. In this latter application, the motor can only drive the lens during the return portion of the mirror drive cycle even though the error may be large which limits the speed at which the system may achieve a focus condition. The prior art also suffers a problem in the area of very small focus errors in that the lens stops even though there is remaining a small error due to the fact that static friction must be overcome in order to move the lens. Another problem encountered in the prior art stems from the fact that mechanical switches tend to bounce upon closure and in systems such as automatic focus where timing of events is critical, the bouncing switch contact may produce errors. Also, the prior art continuous focus proportional controller was limited by the fact that the scan and return cycles were of equal duration. If the active scan cycle could be made a larger portion of the total scan cycle, the angular velocity of the scan mirror may be reduced so as to decrease the focus delay for improved low illumination operation.